Method and device for determining a characteristic of a semicondctor sample

ABSTRACT

The invention relates to a method for determining a characteristic of a semiconductor sample forming a surface. The method comprises the steps: simultaneously illuminating an area on the surface of a semiconductor sample with superimposed exciting light beams with a plurality of wavelengths, modulating the light beam of the different wavelengths with the same frequency, but different phases, selecting a modulation function and its phases in such a way, that the sum of the photon fluxes of all light beams at all times lies within a tolerance range, the tolerance range being considerably smaller than the sum of all photon fluxes, simultaneously phase-dependent measuring of the components of the surface photo voltage caused by the different light beams and determining the characteristic of the semiconductor sample from the relationships between the components and the respective wavelengths. Furthermore a device for carriying out such a method is described.

TECHNICAL FIELD

The invention relates to a method for determining a characteristic of asemiconductor sample forming a surface.

Furthermore the invention relates to a device for determining acharacteristic of a semiconductor sample, comprising: means for mountinga semiconductor sample forming a surface, a plurality of light sourceswith different wavelengths each of the light sources generating a lightbeam, means for superimposing such light beams for the generation of ancombined light beam simultaneously comprising the wavelengths of alllight sources and falling on an area of the surface of the semiconductorsample, modulating means for modulating the light sources with differentmodulating functions, means for measuring the surface photo voltagegenerated in the area of the surface and exhibiting a signaldistribution, and signal evaluation means for determining the signaldistribution components of the surface photo voltage based on thevarious light beams with various wavelengths on the basis of themodulating functions of the various light sources.

The quality of semiconductor materials such as silicon must be monitoredduring the production of material and chips. An important materialparameter is the diffusion length of minority charge carriers. Suchdiffusion length represents the average effective distance, over whichexcess charge carriers diffuse through a semiconductor during their lifetime. This diffusion length is heavily influenced by impurities andcrystal disorders. Therefore the diffusion length of the minority chargecarriers is a suitable measure of the degree of the impurity in thesemiconductor material and of the crysalline perfection. The effectivediffusion length, which is influenced by the surface characteristics hasto be distinguished from the inner diffusion length, which is based onan unlimited sample.

For the quality check of semiconductor materials a quick, nondestructiveand reliable determination of the diffusion length of minority chargecarriers is desireable.

A conventional method of determining of this diffusion length ofminority charge carriers as well as the velocity of the surfacerecombination of semiconductor materials is based on the measurement ofthe surface photovoltage (SPV). The method makes use of the effect, thatafter exciting of a semiconductor sample with light havingphotonenergies which are higher than the energy gap of thesemiconductor, pairs of electrons and holes are formed. A certain amountof such charge carriers can reach the space charge area at the surfaceand are separated causing a voltage drop at the surface. This is thesurface photo voltage SPV. For the determination of the diffusion lengththis surface photo voltage is measured when it is illuminated withdifferent light wavelengths. As the excitation with differentwavelengths causes the generation of excess charge carriers in the innerpart of the semiconductior sample of different depths, the amount ofexcited charge carriers reaching the surface varies with the excitingwavelength. Assuming the excitation is effected with a small injectionlevel, small frequences of the light modulation and diffusion lengths ofthe charge carriers, which are small with respect to the samplethickness a simplified expression for the state of equilibrum can bederived (for example D. Schroder, “Semiconductor Materials and DeviceCharacterization”, John Wiley & Sons, 1990, S.379):${{\Delta\quad V_{SPV}} \propto {\Delta\quad n}} = {\Phi_{eff}\frac{1}{S + \frac{D}{L}}*{\frac{1}{1 + \frac{z}{L}}.}}$

Therein Δn denotes the excess charge carrier-concentration at the edgeof the space charge area, Φ_(eff)=Φ*(1−R) the effective photon fluxentering the semiconductor sample and derived from the incident photonflux Φ taking into account the reflectivity R at the sample surface, Sthe velocity of the frontside surface recombination, z the penetrationdepth of the light, D the diffusion constant of the minority chargecarrier and L the diffusion length of the minority charge carrier of thesample. Using this expression the diffusion length is derived from themeasurement of the voltage change ΔV_(SPV) of the surface voltage due tothe excitation with modulated light of various light sources.

If the velocity of the backward surface recombination cannot beneglected, because the diffusion length of the minority charge carriersreaches the range of the thickness of the semiconductor sample, therelationship can be derived according to the above mentioned book by D.Schroder, page 425 ff:${\Delta\quad V_{SPV}} \propto {( {1 - R} )\Phi\quad\frac{S_{f}}{C}*{\frac{1}{1 - {z^{2}/L^{2}}}\begin{bmatrix}{{( {{z\quad\frac{S_{b}}{D}} - 1} )\begin{pmatrix}{{\cosh\quad\lbrack {T/L} \rbrack} -} \\{\exp\quad\lbrack {{- T}/z} \rbrack}\end{pmatrix}} +} \\{( {\frac{z}{L} - \frac{S_{b}L}{D}} )\quad{\sinh\quad\lbrack {T/L} \rbrack}}\end{bmatrix}}}$ whereinC = (S_(f)S_(b)L/D + D/L)  sinh   [T/L] + (S_(f) + S_(b))  cosh   [T/L].

R denotes the reflectivity at the frontside of the semiconductor sample,φ the incident photon flux, S_(f) and S_(b) the front and rear velocityof the surface recombination, z the penetration depth of the light, Lthe diffusion length of the minority charge carriers, D the diffusionconstant of the minority charge carriers and T the thickness of thesemiconductor sample.

PRIOR ART

Known methods based on the measurement of the surface photo voltage usethe above first mentioned, simplified relationship.

With one of such methods the level of the generated surface photovoltage is kept constant. The principle of such a method is described ina publication by Goodman et al. “A metghod for the measurement of shortminority carrier diffusion lengths in semiconductors” in the Journal,“J. Appl. Phys.” Vol. 33, p.2750, 1961.

The photon flux and the surface photo voltage are measured withdifferent wavelengths according to different photo energies. The photonflux is changed in such a way, that a constant surface photo voltage isobtained for each wavelength. The different photon fluxes obtained withdifferent photon energies are plotted against the reciprocal absorbtioncoefficient. This graph is then linearity extrapolated and the point ofintesection of the extrapolated graph with the abscissa is determinedfor the determination of the diffusion length of the minority chargecarriers.

This is also described in the U.S. Pat. No. 5,177,351 A as the state ofthe art.

In a different kind of such a method the photon flux is kept constant.

Such a method is already described as prior art in the U.S. Pat. No.5,663,657 A.

The U.S. Pat. No. 5,025,145 A describes such a method, wherein first theinduced photo voltage is measured for different photon fluxes to ensurea linear relationship between the surface photo voltage and the photonflux. Then the photo voltage is measured for a number of selected photonenergies, i.e. wavelengths with light with a constant photon flux havinga value in the linear range. The surface photo voltages obtained in sucha way having monotonously increasing photon energies are plotted as afunction versus the reciprocal values of the absorbtion coefficientsaccording to the given photon energies. The diffusion length of theminority charge carriers is determined by extrapolation for determiningthe reciprocal absorbtion coefficient for a (modulated) surface photovoltage equal to zero.

Both methods are based on the above mentioned simplified model requiringa linear relationship between the photon flux required for a constantsurface photo voltage or the surface photo voltage, respectively, on onehand and the penetration depth of the light. The use of such a modelallows the determination of the frontside surface recombination and thediffusion length. The influence on the rear side of the semiconductorsample is normally ignored or the results are corrected in an againsimple way using a factor depending on the thickness of thesemiconductor sample.

As the optical excitation with different wavelengths is carried outsequentially, both methods are relatively slow. Furthermore, the“CMSPV”-method operating with constant surface photo voltage has thedisadvantage that for each wavelength the photon flux hast to be newlyadjusted in order to obtain the given photo voltage. Even if this iscarried out by a seperate control circuit, this causes further slowdown.The speed of the measurement is of crucial importance, if the surface ofa semiconductor sample needs to be scanned point by point for aplurality of points.

Furthermore the sequential measurements of the surface photo voltage forthe different wavelengths cause systematic errors. The state of thesurface is changed by the measurement itself. Thereby the accuracy ofthe method is limited. The velocity of the surface recombination evendepends on the intensity of the optical excitation, i.e. on the photonflux incident on the surface of the semiconductor sample. As at least aportion of the surface states exhibit a relatively slow recombination, ahysteresis effect occurs. This violates the assumption of the simplifiedmodel that all data points for the different wavelengths are obtainedunder the same surface conditions.

The WO 00/02058 (=DE 198 31 216 A1) describes a method for determiningthe dependency of the surface photo voltage on the wavelength. Thereinthe wavelength is periodically changed over a wavelength range at acertain frequency. The surface photo voltage changing in accordancetherewith is measured. From the obtained measured signal the componentswith a plurality of frequencies are determined by means of fouriertransformation. These components are corrected with respect to thefrequency response and phase response of the measurement set up. Fromthe such determined components the surface photo voltage is determinedby signal processing.

From the U.S. Pat. No. 6,512,384 B1 a method for the determination ofthe diffusion length of minority charge carriers is known, wherein thesurface photo voltage at each investigated position of the semiconductorsample measurements are carried out simultaneously for a plurality ofwavelengths of the exciting light. In such known method the surface ofthe semiconductor sample is excited with a light beam composed ofsuperimposed photon fluxes with different wavelengths. From the measuredsurface photo voltage the components with the different modulationfrequencies are filtered. Thereby the speed of the measurement isconsiderably increased.

This known method, however, does not avoit the errors caused by thedependency of the surface recombination on the intensity. As amodulation with different frequencies causes a variation of the entirephoton flux between zero and the total amount of all photon fluxes ofthe superimposing wavelength light sources, the state of the surfacealso then considerably changes during the measuring process.

DISCLOSURE OF THE INVENTION

It is an object of the invention, to carry out the measurement at eachpoint of a semiconductor sample with a method for the determination ofthe diffusion length of minority charge carriers or equivalentcharacteristics of a semiconductor sample quicker and more accurate.

More precisely it is an object of the invention to ensure essentiallythe same surface states for a method of such kind wherein theinvestigated point is excited with light of different wavelengths andwherein the surface photo voltages obtained for the differentwavelengths are determined.

According to the invention this object is achieved with a method of theabove mentioned kind with the method steps:

-   -   (a) simultaneously illuminating an area on the surface of a        semiconductor sample with superimposed exciting light beams with        a plurality of wavelengths,    -   (b) modulating the light beam of the different wavelengths with        the same frequency, but different phases,    -   (c) selecting a modulation function and its phases in such a        way, that the sum of the photon fluxes of all light beams at all        times within a tolerance range, the tolerance range being        considerably smaller than the sum of all photon fluxes,    -   (d) simultaneously phase-dependent measuring of the components        of the surface photo voltage caused by the different light beams        and    -   (e) determining the characteristic of the semiconductor sample        from the relationships between the components and the respective        wavelengths.

Based on this method a device for determining a characteristic ofsemiconductor sample, comprising: means for mounting a semiconductorsample forming a surface, a plurality of light sources with differentwavelengths each of the light sources generating a light beam, means forsuperimposing such light beams for the generation of a combined lightbeam simultaneously comprising the wavelengths of all light sources andfalling on an area of the surface of the semiconductor sample,modulating means for modulating the light sources with differentmodulating functions, means for measuring the surface photo voltagegenerated in the area of the surface and showing a signal distribution,and signal evaluation means for determining the signal distributioncomponents of the surface photo voltage based on the various light beamswith various wavelengths by means of the modulating functions of thevarious light sources, as shown in the U.S. Pat. No. 6,512,384 B1, theinvention provides modulation means operating with the same frequenciesfor all light sources, the modulation means operating with phase shiftsamong the different light sources and the modulation functions and itsphases being selected in such a way that the sum of all photon fluxeslies within a tolerance range, which is considerably smaller than thesum of the photon fluxes.

With the method according to the invention and the device according tothe invention the measurements are carried out simultaneously for allexciting wavelengths as in the U.S. Pat. No. 6,512,384 B1, so that themeasuring time is shortened. However, the difference between thecomponents of the obtained signal distribution of the surface photovoltage based on the different wavelengths are distinguished not bytheir frequency which is used to modulate the different wavelengths orlight sources, but their modulation phase. The modulation frequency isthe same for all wavelengths. Thereby it is possible to select thephases of the different modulations in such a way that the entire photonflux is approximately constant. Therefore the measurement is alwayscarried out under the same conditions in this respect. The errorsoccuring due to variations of the photon flux for different modulationfrequencies are avoided.

Further aspects of the invention are subject matter of the subclaims.

An embodiment of the invention is described below in greater detail withreference to the accompanying drawings.

FIG. 1 is a block diagram and scematically shows a device for checking asemiconductor sample with modulated light of different wavelengths andfor measuring the surface photo voltage.

FIG. 2 scematically shows the set up of a device for illuminating thesemiconductor sample with modulated light of different wavelengths andfor measuring the surface photo voltage.

FIG. 3 shows the distribution of the surface photo voltages generatedwith a device according to FIG. 2 by three light beams of differentwavelengths modulated with different phases.

In FIG. 1 numeral 10 denotes a computer for data processing andcontrolling the computer 10 controls an illumination unit 12. Theillumination unit 12 generates, as described below, three light beamswith different wavelengths, which are modulated with the same frequencybut with different phases. The light beams and their modulation aremonitored by an excitation monitor 14. The excitation monitor providesthe distribution of the photon fluxes of the three light beams. Suchdistributions are switched on the computer 10 for the data processing. Atesting probe 16 measures the surface photo voltage generated byexcitation with the light beams at one point of a semiconductor sample18. The distribution of this surface photo voltage is fed to thecomputer 10 via a data receiving unit 20. The semiconductor sample 18 ismounted on a sample table 22 which is moveable under the control of thecomputer 10 for scanning the surface of the semiconductor sample 18.

The set up is schematically shown in FIG. 2. The sample 18 isilluminated at a measuring point by a plurality of light sources. InFIG. 2 three such light sources 24, 26, and 28 are shown. The lightsources 24, 26 and 28 form the illumination unit 12 in FIG. 1. The lightsources 24, 26 and 28 are preferably laser diodes or luminescent diodes.The light sources emit light beams 30, 32, 34 with differentwavelengths. The light beams 30, 32, 34 are each collimated by acollimating optics 36, 38 and 40, respectively, on end surfaces offibers 42, 44 and 46, respectively. A fiber coupler 48 superimposes thelight beams guided in the fibers 42, 44 and 46. The superimposing lightbeams are then guided through a fiber 50 to a measuring point 52 on thesurface of the semiconductor sample 18.

The testing probe 16 measures the surface photo voltage at the measuringpoint 52 on the surface of teh semiconductor sample 18. Such testingprobes are well known to the person skilled in the art and are,therefore, not described here in greater detail.

A modulator assembly 54 modulates the light emission of the lightsources 24, 26 and 28 and, thereby, the photon flux of the light beam30, 32 and 34, respectively. The modulation is carried out periodicallywith the same frequency for all light beams 30, 32 and 34. Themodulation function, however, are phase shifted amongst each other. FIG.3 shows a preferred form of a modulation function and the respectivedistribution of the surface photo voltage generated thereby. Themodulation functions essentially correspond to oneway-rectifyed sinefunctions, i.e. the positive half-waves of the sine function. Themodulation functions are essentially zero between the positivehalf-waves. In FIG. 3 numeral 56 denotes the modulation function or thecorresponding surface photo voltage of the light source 24. Numeral 58is the modulation function of the light source 26 and numeral 60 denotesthe modulation function of the light source 28. The three modulationfunctions 56, 58 and 60 are phase-shifted by 120° relative to thefrequency of the modulation functions. For any number n of light sourcesthe phase shifts preferably is 2 π/n. In FIG. 3 numeral 62 denots thesum of the photon fluxes or surface photo voltages, respectively. It canbe seen, that the photon flux of the superimposed light beams, i. e. thesum of the photon fluxes of all light beams, at all times lies within atolerance range, which is considerably smaller than the sum of thephoton fluxes. Thereby approximately constant illumination conditionsfor all wavelengths are obtained at the measuring point 52. By theselection of the modulation functions 56, 58, 60 a rest wave structureis obtained for the sum of the photon fluxes and the surface photovoltage SPV. This facilitates the attribution of the differentcomponents of the surface photo voltage to the individual light sources24, 26, 28 and thereby to the different wavelengths.

The modulation functions are fed to a quick A/D-converter or a plurallock-in-circuit 70 by a modulator assembly 54 with lines 64, 66 and 68.The lock-in-circuit 70 receives the surface photo voltage from a voltageamplifier. The lock-in-circuit 70 forms a part of the data receivingunit 20 and provides signals at the computer 10 according to thedifferent modulated light sources 24, 26 and 28 and the components ofthe surface photo voltage caused by the corresponding wavelengths of theexciting light.

A constant, non-modulated photon flux is guided to the measuring point52 from a light source 74 and a fiber 76 or a bundle of fibers. Thisphoton flux only provides a non-modulated component of the surface photovoltage. It does not generate a modulated signal with the modulationfrequency and therefore does not disturb the result of the measurement.This non-modulated photon flux can be used to saturate surface states ortraps for minority charge carriers over the entire thickness of thesemiconductor sample 18.

A photo detector 78 receives light through a bundle 80 of fibers whichis reflected at a measuring point 52 from the surface of thesemiconductor sample 18. From the photon flux incident from the fiber 50and the photon flux reflected on the surface of the semiconductor sample18 the effective photon flux actually entering the semiconductor samplecan be determined. The signal of the photodetector 78 is fed to thelock-in-circuit 70 through an amplifier.

A temperatur sensor 79 measures the temperature of the semiconductorsample 18. The signal of the temperatur sensor 79 is fed to the computer10 or the lock-in-circuit 70, respectively, through an amplifier 81.

The described assembly operates as follows:

The exciting photon flux φ_(exc)(t) entering the semiconductor sample is${\Phi_{exc}(t)} = {\Phi_{bias} + {\sum\limits_{j = 1}^{n}{\Phi_{j}*{M(t)}*{\exp( {i\quad\varphi_{j}} )}*{T_{reflex}( \lambda_{j} )}}}}$wherein φ_(bias) is the photon flux of the background light, φ_(j) andφ_(j) representing the amplitude and the phase shift of the light beamat the wavelength j, M(t) is the modulation function used for modulatingthe light sources 24, 26, and 28 and T_(reflex)(λ_(j)) the reflectioncorrection for the j^(th) wavelength due to the reflection of thesurface of the semiconductor sample 18. The letter i denotes theimaginary number.

The reflection correction T_(reflex)(λ_(j)) is calculated from thesimultaneously measured portion of the exciting light which is reflectedfrom the surface of the semiconductor sample 18 into the bundle offibers 80. At the end of the bundle of fibers 80 a photo detector with alarge detection area detects the light. The electrical signal generatedfrom the photo detector 78 is finally fed to the computer 10 through anamplifier 82. By suitable adjustment of the transmission function of thebundle 80, the characteristics of the photo detector 78 and thereflection of the optics of the SPV testing probe head before theautomatized measuring process the actual reflected light at eachmeasuring point 52 of the semiconductor sample 18 at each usedwavelength can be determined and can be used to correct the photon flux.The exciting photon flux φ_(exc), the photon flux penetrating thesemiconductor sample, is computed from the incident photon flux φ_(j)for each wavelength.

The simultaneous measurement of the reflected light renders the assemblyand the method independent from the theoretical photon flux corrections.

In a modified embodiment the light reflected at the surface of thesemiconductor sample can be directly measured within the testing probehead by means of a suitable beam splitter in the form of a partial beamderived from the main beam.

The amplitudes and phases of the signals of surface photo voltagesgenerated by exciting with n wavelengths are simultaneously measuredwith the testing probe 16 in connection with the data receiving unit 20and fed to the computer 10 for further signal processing. the datareceiving unit 20 is a quick analogue-digital-converter, which scans thesignal of the surface photo voltage with a scanning rate which is higherthan 2 nf, wherein f is the modulation frequency. In order to improvethe signal-to-noise ratio and to ensure that all components of thedifferent wavelengths are exposed to the same scanning conditions, thesignal is detected over a plurality of the modulation frequency andstored. After the detection of the data the signal distribution forextracting the diffusion length is processed. The signal distribution issmoothed by suitable means, for example a digital filter applied to thesignal in the computer program, to reject noise components not belongingto the surface photo voltage. The signal can also be averaged zu asignal distribution, in such a way that finally a final signaldistribution is obtained with a window of one modulation period, i.e. awindow with a time interval of 1/f.

The computer 10 is fed with a signal representing the change of thesurface photo voltage by excitation with the superimposed, modulatedlight beams 24, 26, 28:${\Delta\quad{V_{SPV}(t)}} = {\sum\limits_{j = 1}^{n}{\Phi_{j}*{M(t)}*{\exp( {{\mathbb{i}}\quad\varphi_{j}} )}*{T_{reflect}( \lambda_{j} )}*{T_{SPV}( \lambda_{j} )}}}$

Therein M(t) is the signal distribution of the driver signal of themodulator assembly which is used to modulate the light source 24, 26 and28, i.e the j=1^(st), j=2^(nd) and j=3^(rd) light source, i.e. the abovementioned modulation function, φ_(j) denotes the phase shift of thej^(th) exciting beam 30, 32, 34, i.e. the j=1^(st), j=2^(nd) andj=3^(rd) exciting beam, T_(reflex) denotes the correction functiontaking into account the light reflected at the surface of thesemiconductor sample 18 and T_(SPV) is the modification by which theeffect caused by the surface photo voltage itself is taken into account.The transmission function T_(reflex)(λ_(j)) resulting from the lightreflected for each wavelength measured with the photo detector 78 asdescribed above is a correction factor for the amplitude of the j^(th)excitation beam. In the embodiment of FIG. 2 the distribution M(t) ofthe modulation function is provided by the modulator assembly 54controlling the light sources according to such a synthetically derivedfunction. The distribution M(t) also can be measured from the lightemitted by the light sources 24, 26 and 28.

The effect of the surface photo voltageT_(SPV)(λ_(j))=A_(SPV)(λ_(j))exp(iΨ_(j)) represents a change of theamplitude and the phase of each component with the correspondingwavelength. The relationship between the exciting photon flux and thechange of the surface photo voltage caused thereby is given by a real“adjustment factor” A_(SPV) and a phase shift Ψ. The surface of thesemiconductor sample operates in a similar way as a time functionelement with capacitor and resistor. The above equation can be writtenas:${\Delta\quad{V_{SPV}(t)}} = {\sum\limits_{j = 1}^{n}{\Phi_{j}*{M(t)}*{\exp( {{\mathbb{i}}\quad\varphi_{j}} )}*{T_{reflex}( \lambda_{j} )}*{A_{SPV}( \lambda_{j} )}*{\exp( {{\mathbb{i}}\quad\psi_{j}} )}}}$

By adapting the amplitudes A_(SPV)(λ_(j)) and the phase shiftsexp(iΨ_(j)) the signal distribution resulting from the superposition ofthe modulation frequencies with the corresponding photon fluxes of thedifferent wavelengths can be adapted to the measured signal ΔV_(SPV) ofthe surface photon voltage. Practically the graphs 56, 58 and 60(φ_(j)M(t)) corrected with respect to the wavelength dependentreflectivity, are changed in amplitude and phase according to a certainalgorithm until the sum of the obtained signal distribution coincideswith the measured distribution of the modulation ΔV_(SPV) of the surfacephoton voltage. The graph 90 in FIG. 3 shows the adaption to the thusobtained graph distribution. The graph 92 shows the measureddistribution of the surface photon voltage.

The thus obtained values for the phase shift Ψ_(j) can be used tocorrect the amplitude of the surface photon voltage SPV with respect tothe dependency of the modulation frequency. (Nakhmanson “Solid StateElectronics” 1975, Vol 18, P. 617).

The corrected values A*_(SPV)(λ_(j)) directly are the relationshipsbetween the photon fluxe and the amplitude of the surface photon voltageSPV for the j^(th) wavelength. Thereby the diffusion length L of theminority charge carriers can be determined for each measuring pointdirectly from A*_(SPV)(λ_(j)).

Two computation methods can be applied:

The approximation for short diffusion lengths uses the above mentionedrelationship${{\Delta\quad V_{SPV}} \propto {\Delta\quad n}} = {\Phi_{eff}\frac{1}{S + \frac{D}{L}}*\frac{1}{1 + \frac{z}{L}}}$

This relationship is valid, if the diffusion length is shorter thanabout half the thickness of the sample. If two wavelengths are assumedthe values A₁ and A₂ represent the relationships ΔV_(SPV−1)/φ₁ andΔV_(SPV−2)/φ₂. L can be derived therefrom using the above equation as${L = {( {{\frac{A_{2}}{A_{1}}z_{2}} - z_{1}} )/( {1 - \frac{A_{2}}{A_{1}}} )}},$wherein z_(j) are the penetration depths at the j^(th) wavelength.

As mentioned above this approximation for short wavelengths is onlyvalid for a relative to the diffusion length large thickness of thesemiconductor sample. However, today high quality semiconductor materialhas a diffusion length comparable with the thickness of thesemiconductor sample or even larger than that. In such a case the effectof the rear surface recombination has to be taken into account byfitting the results into the above mentioned more general equation. Atfirst the signal ΔV_(SPV) is normalized to for example the signaloccurring at the shortest wavelength λ_(min). This normalizationprovides:${\Delta\quad V_{SPV}{norm}\quad(\lambda)} = \frac{\Delta\quad{V_{SPV}(\lambda)}}{\Delta\quad{V_{SPV}( \lambda_{\min} )}}$

By such a normalization the above given prefactor C is eliminated. Withthe normalized signal (the adding of the term “norm” being omitted)results in:${\frac{\Delta\quad{V_{SPV}(\lambda)}}{\Phi_{eff}(\lambda)}/\frac{\Delta\quad V_{SPV}( \lambda_{1} )}{\Phi_{eff}( \lambda_{1} )}} \propto {\frac{L^{2}}{L^{2} - z^{2}}\begin{bmatrix}{{( {{z\quad\frac{S_{b}}{D}} - 1} )( {{\cosh\quad\lbrack \frac{T}{L} \rbrack} - {\exp\quad\lbrack \frac{- T}{z} \rbrack}} )} +} \\{( {\frac{z}{L} - \frac{S_{b}L}{D}} )\quad{\sinh\quad\lbrack \frac{T}{L} \rbrack}}\end{bmatrix}}$R is the reflectivity on the front side of the semiconductur sample 18,S_(b) is the velocity of the surface recombination on the rear side, zis the penetration depth of the light, L is the diffusion length of theminority charge carriers, D is the diffusion constant of the minoritycharge carrier and T is the thickness of the semiconductor sample 18.

If n values A₁, A₂, . . . A_(n) are used (i.e. ΔV_(SPV−1)/φ₁,ΔV_(SPV−2)/φ₂, . . . ΔV_(SPV−n)/φ_(n)), as they result from themeasurement for the ratios Γ_(j)=A_(j)/A₁, i.e. for the values A_(j)normalized against A₁ it is obtained$\Gamma_{j} \propto {\frac{L^{2}}{L^{2} - z_{j}^{2}}*{\lbrack {{( {{z_{j}\frac{S_{b}}{D}} - 1} )( {{\cosh\quad\lbrack \frac{T}{L} \rbrack} - {\exp\lbrack \frac{- T}{z_{j}} \rbrack}} )} + {( {\frac{z_{j}}{L} - \frac{S_{b}L}{D}} )\quad{\sinh\quad\lbrack \frac{T}{L} \rbrack}}} \rbrack.}}$

If L and S_(b) are taken as free adaption parameters the diffusionlength and the rear velocity of the surface recombination can be derivedfrom the n−1 equations resulting from the above expression. This methodis not dependent on any assumptions about the rear velocity of thesurface recombination. As an additional parameter, i.e. S_(b), isdirectly derived from the measurement at least three wavelengths areneeded for this method.

1. Method for determining a characteristic of a semiconductor sampleforming a surface with the steps: (a) simultaneously illuminating anarea on the surface of a semiconductor sample with superimposed excitinglight beams with a plurality of wavelengths, (b) modulating the lightbeam of the different wavelengths with the same frequency, but differentphases, (c) selecting a modulation function and its phases in such away, that the sum of the photon fluxes of all light beams at all timeslies within a tolerance range, the tolerance range being considerablysmaller than the sum of all photon fluxes, (d) simultaneouslyphase-dependent measuring of the components of the surface photo voltagecaused by the different light beams and (e) determining thecharacteristic of the semiconductor sample from the relationshipsbetween the components and the respective wavelengths.
 2. Methodaccording to claim 1, characterized in that the characteristic to bedetermined is the diffusion length of minority charge carriers. 3.Method according to claim 2, characterized in that the determination ofthe diffusion length of the minority charge carriers is determined byfitting value pairs of surface photo voltage ΔV_(SPV) and wavelengths λor the penetration depth z of the light into the equation$\frac{\frac{\Delta\quad{V_{SPV}(\lambda)}}{\Phi_{eff}(\lambda)}}{\frac{\Delta\quad{V_{SPV}( \lambda_{1} )}}{\Phi_{eff}( \lambda_{1} )}} \propto {\frac{L^{2}}{L^{2} - z^{2}}\lbrack {{( {{z\frac{S_{b}}{D}} - 1} )( {{\cosh\lbrack \frac{T}{L} \rbrack} - {\exp\lbrack \frac{- T}{z} \rbrack}} )} + {( {\frac{z}{L} - \frac{S_{b}L}{D}} ){\sinh\lbrack \frac{T}{L} \rbrack}}} \rbrack}$wherein S_(b) is the velocity of the rear surface recombination, L isthe diffusion length of the minority charge carriers, D is the diffusionconstant of the minority charge carriers and T is the thickness of thesemiconductor sample.
 4. Method according to any of claims 1 to 3,characterized in that the intensity distribution of the different lightbeams is measured and used for determining the respective compontents ofthe surface photo voltage.
 5. Method according to any of claims 1 to 4,characterized in that additionally the photon flux reflected from thesurface of the semiconductor sample is measured for the determination ofthe effective photon flux really penetrating the semiconductor. 6.Method according to any of claims 1 to 5, characterized in that thetemperature of the semiconductor is measured for taking into account thedependence of the wavelength dependent penetration depth of the photonflux on the temperature.
 7. Method according to any of claims 1 to 6,characterized in that (a) the modulation of each light beam is carriedout with a modulation function essentially corresponding to the positivehalf-waves of a sine function, and (b) the modulation function of thedifferent light beams are phase shifted by essentially 2 π/n relative tothe modulation frequency which is the same for all light beams, whereinn denotes the overall sum of the superimposed light beams.
 8. Methodaccording to any of claims 1 to 7, characterized in that thesemiconductor is step-by-step moved relative to the superimposed lightbeams.
 9. Device for determining a characteristic of a semiconductorsample, comprising: (a) means (22) for mounting a semiconductor sample(18) forming a surface, (b) a plurality of light sources (24, 26, 28)with different wavelengths each of the light sources generating a lightbeam (30, 32, 34), (c) means (42, 44, 46, 48, 50) for superimposing suchlight beams for the generation of an combineded light beamsimultaneously comprising the wavelengths of all light sources andfalling on an area of the surface (52) of the semiconductor sample (18),(d) modulating means (54) for modulating the light sources (24, 26, 28)with different modulating functions, (e) means (16) for measuring thesurface photo voltage generated in the area of the surface and showing asignal distribution, and (f) signal evaluation means (70) fordetermining the signal distribution components of the surface photovoltage based on the various light beams (30, 32, 34) with variouswavelengths by means of the modulating functions of the various lightsources (24, 26, 28), characterized in that (g) modulation means (54)operating with the same frequencies for all light sources, (h) themodulation means (54) operate with phaseshifts among the different lightsources, and (i) the modulation functions and its phases are selected insuch a way that the sum of all photon fluxes lies within a tolerancerange, which is considerably smaller than the sum of the photon fluxes.10. Device according to claim 8, characterized in that (a) modulationmeans are provided for modulating each of the light beams (24, 26, 28)is modulateable by modulating means with a modulation function (56, 58,60) essentially corresponding to the positive half-waves of a sinefunction, and (b) the modulation function (56, 58, 60) of the differentlight beams (30, 32, 34) are phase shifted by essentially 2 π/n relativeto the modulation frequency which is the same for all light beams (30,32, 34), wherein n denotes the overall sum of the superimposing lightbeams (30, 32, 34).
 11. Device according to claim 9 or 10, characterizedin that the characteristic to be determined is the diffusion length ofminority charge carriers.
 12. Device according to any of claims 9 to 11,characterized in that the light sources (30, 32, 34) are laser diodes.13. Device according to any of claims 9 to 11, characterized in that thelight sources (30, 32, 34) are luminescence diodes.
 14. Device accordingto claim 12 or 13, characterized in that the means for superimposing theligth beams are provided with a fiber coupler (48) wherein the differentlight beams can be coupled through entrance side fibers (42, 44, 46) andwhich is connected to an exit side fiber (50) receiving the superimposedlight beam and guiding it to an illuminated area (54) on the surface ofthe semiconductor sample (18).
 15. Device according to any of claims 9to 14, characterized by means (78, 80) for measuring the portion of thelight beam reflected at the surface of the semiconductor sample (18).